1. Field of the Invention
The invention relates to floating underwater support structures that can support structures placed above or on the surface of a body of water, and can maintain a stable, substantially upright orientation in shallow and deep water.
2. Description of Related Art
Many processes, including various types of energy exploration and production, require that a floating structure be placed permanently or semi-permanently in a body of water. The manner in which these floating structures are placed and supported depends on the weight and size of the structure, the depth of the water, the wind and sea conditions at the site, and other such considerations.
For heavy equipment that is to be located in deep water, the tension leg platform (TLP) is one conventional solution. In a TLP, a platform above the surface of the water is rigidly attached to a buoyant member located below the surface of the water. The buoyant member, in turn, is anchored by one or more tension cables, also called tension legs, that are tethered to seabed anchors. The buoyant force provided by the submerged buoyant member maintains the tension legs in tension. TLPs have long been used, for example, to support oil production platforms in deep water.
Despite their uses, TLPs have certain disadvantages. Because a TLP's center of buoyancy is typically below the structure's center of gravity, the structure tends to capsize without tension in the tension legs. This is not typically a problem in deep water, where the structure can be designed to minimize the influence of the waves and maintain tension in the legs. However, if a TLP is placed in shallow water, it is more affected by wave loads, which could introduce compression into the tension legs and risk capsizing the TLP.
Thus, TLPs are not ideal for supporting lighter structures, such as wind turbines, that are to be placed closer to shore in shallow or moderately deep water. Unfortunately, other solutions for supporting lighter structures in shallow and moderately deep water are relatively few. Moreover, support structures that work well in shallow water are often hard to implement in deeper water, and as with TLPs, support structures intended for deeper water often do not work well in shallow water. For example, a spar buoy (i.e., a tall, slender floating structure) has too large a draft to function in shallow water.
One approach for supporting a structure such as a wind turbine in shallow or moderately deep water is to use a structure that relies on a large surface area to float on the surface of the water, much in the way that a conventional ship floats. However, that approach often results in a structure that is large and heavy. Another approach is to build a platform or support with rigid legs that reach all the way to the sea floor, but that approach may only be feasible in relatively shallow water.
Ultimately, the lack of good solutions for supporting floating structures has resulted in structures like wind turbines being placed primarily in shallow water (e.g., 35 m or less). There are a few prototype structures for supporting wind turbines in deep water (e.g., 200 m or more). There are a limited number of economically and technically feasible choices for supporting wind turbines and other structures in water that is, for example, 35-200 m deep, yet there are many otherwise suitable sites with water depths in that range.